Clock signal is the most important electrical signal. It directly links to the thing called “time”, which is used to coordinate events inside electronic world. Clock signal is generated from frequency source. Frequency source is found in electronic systems from military, metrology, industrial, consumer, communication networks, automotive, power grid, banking and scientific project. Taking telecommunication industry for example, GPS provided timing supports a variety of applications: a frequency source of 1·10−11 accuracy functions as a primary reference clock for Switched Telephone Networks (STN); time synchronization of microsecond accuracy is used in Cellular Telephone System to synchronize cell sites for allowing seamless switching; Network Time Protocol (NTP) uses millisecond accuracy to support application level usage of accurate time while Precise Timing Protocol (PTP) uses sub-microsecond. IP based applications like streaming audio & video need solid frequency sources for delivering good timing information.
Inside electronic system, clock derives time from frequency. A typical clock system includes a frequency source and a counting circuit (for counting, setting and synchronization). There are two concepts when clocks are used to coordinate events among systems: synchronization and syntonization. Clocks are synchronized when they are agreed in time. Clocks are syntonized when their oscillators have the same frequency. No clock can ever keep perfect time since all oscillators exhibit random and systematic errors. Clock error can be expressed in (1) where T(t) is the time difference between two clocks at time t after synchronization, T0 is synchronization error at t=0, R(t) is the frequency difference between the two clocks' oscillators, ε (t) is the error due to random fluctuations.T(t)=T0+∫0iR(t)dt+ε(t)  (1)
When compared with ideal frequency source, the frequency value of a real source always deviates from the specified value. This imperfection, which is caused by the combined effect of manufacture error, temperature variation, aging, loading shift, is often referred as frequency instability. Commercially there are various types of frequency sources available for choosing, from a few cents XO to a few hundred dollars TCXO (temperature-control crystal) to a few thousand dollars OCXO (oven-controlled crystal). Their performances vary greatly: from XO's tolerance of 10˜100 ppm, to TCXO's˜1 ppm, to OCXO's˜0.1 ppm.
Equation (1) relates frequency error to time error. In many cases, frequency error is caused by frequency instability. In this application the term frequency accuracy is used to represent, in the frequency value of a given frequency source, the difference between actual value and the value specified in its specification. The term frequency stability is used to describe its capability of maintaining its frequency accuracy under the influence of disturbances. The major causes of frequency instability are: temperature step, aging, vibration, shock, oscillator on/off switching and etc. In high performance system, TCXO, or even OCXO, are adopted. Those high quality sources usually have much better frequency stability than low end crystals. The drawback however is the extreme high cost. Out of the causes mentioned above, the frequency instabilities due to temperature and aging can potentially be compensated. Commercial examples for compensating temperature induced instability are available. An example is given in [1] where temperature sensor is used to report temperature reading and built-in fractional-N or integer-N PLL is used to counteract the corresponding frequency deviation. Its resolution of frequency correction is in the range of hundreds Hz. The compensation however is for the particular functional clock (the RF carrier) and the circuit is custom designed. It is hard to be applied to general case.
Time-Average-Frequency Direct Period Synthesis (TAF-DPS) is an emerging frequency synthesis technique [2-3]. Its distinguishing features are small frequency granularity (termed as arbitrary frequency generation) and fast frequency switching (termed as instantaneous frequency switching). Experiment evidence is available to support the claim that its frequency granularity can reach the level of a few ppb [4]. In present application, TAF-DPS is used to develop a scheme for compensating frequency error and improve frequency source's frequency accuracy and frequency stability.
In 2008, a novel concept, Time-Average-Frequency (TAF), is proposed [2]. It removes the constraint that all the cycles in a clock pulse train have to be equal in their length-in-times. As a result, a TAF clock signal can be created by using two, or more, types of cycles. Small frequency granularity can be obtained by adjusting the weighing factor in very fine step. Fast frequency switching is accomplished through directly synthesizing the length of each individual clock pulse. Together, a new technology, Time-Average-Frequency Direct Period Synthesis, is emerged [3]. Its aim is to provide the features of arbitrary frequency generation and instantaneous frequency switching to chip designers and system users.
The important features of arbitrary frequency generation and instantaneous frequency switching enabled by TAF-DPS clock generator are extremely useful for future electronic system designs. They are the enabler for future innovations. In current application those features are used to construct a scheme for improving frequency source's frequency accuracy and frequency stability by compensating frequency error due to temperature deviation, component aging and other disturbances.
This “Discussion of the Background” section is provided for background information only. The statements in this “Discussion of the Background” are not an admission that the subject matter disclosed in this “Discussion of the Background” section constitutes prior art to the present disclosure, and no part of this “Discussion of the Background” section may be used as an admission that any part of this application, including this “Discussion of the Background” section, constitutes prior art to the present disclosure.